This invention relates generally to fuel injection systems for diesel engines and more particularly to engine driven unit injectors defining a metering chamber and operable in a pressure-time method of fuel metering.
U.S. Ser. No. 282,629 filed July 13, 1981 discloses a fuel system having a unit injector which includes a single electrically controlled solenoid that controls each of the functions of timing and the quantity of fuel that is permitted to enter a metering chamber defined within the unit injector prior to the moment of initiating fuel injection into the diesel engine. The control valve is controlled by an electronic control unit (ECU) of a known variety such as that described in Ser. No. 945,988 filed Sept. 25, 1978 which is incorporated herein by reference. The Sisson et al fuel injector includes a fixed yet adjustable orifice within the fuel line leading to the metering chamber. The incorporation of the orifice within the fuel line or passage permits the regulation of the amount of fuel entering the metering chamber in proportion to the pressure drop across the adjustable orifice and the time during which fuel is permitted to flow into the metering chamber. Consequently the above utilized method of fuel metering has been referred to as a pressure-time method of fuel metering. As disclosed in U.S. Ser. No. 282,629, when the pumping piston is retracted quickly a vapor is created in the timing chamber which is located between the pumping piston and a reciprocal metering piston. The metering piston floats freely within its sleeve or bore while a controlled quantity of fuel is allowed to enter the metering chamber which is defined between the metering piston and the lower end of the sleeve or bore. To prevent the metering piston from rising excessively, that is, from moving toward the pumping piston, it has been suggested to incorporate mechanical stops to limit the maximum extent of the upward motion of the metering piston. If the metering piston is permitted to rise in an uncontrolled manner, it may contribute to premature fuel injection into the engine and permit an excessive amount of fuel to enter the metering chamber which upon subsequent injection into the engine will contribute to excessive smoke and noxious exhaust fumes. The present invention utilizes a unique cam profile that allows the cam driven pumping piston to be stopped during the retraction portion of its operating cycle. In addition, the present invention utilizes the pumping piston as a physical stop thereby preventing excess travel of the metering piston during the metering portion of operation.
The major disadvantages of including a mechanical stop such as a pin and the like for limiting the upward motion of the metering piston are that the manufacturing cost of the piston and its housing are increased; the reliability of a mechanical pin is questionable since they are prone to break and the incorporation of pins within a unit injector normally requires that the size of the metering piston be increased thereby increasing the overall size of the injector. In addition, the incorporation of a pin may require that the metering chamber be increased substantially which may have a degrading effect upon performance because of the increased compressible volume of the fuel within the now larger metering chamber.
Accordingly, the present invention comprises:
a housing having a bore, a pumping piston situated within the bore; means for reciprocating the pumping piston in synchronism with the engine; a metering piston situated within the bore below the pumping piston; a timing chamber situated between the pistons; a metering chamber situated below the metering piston; a nozzle connected in fluid connection with and located remote from the metering chamber; control valve means in communication with (a) a source of fuel, (b) the timing chamber for opening and closing the timing chamber at a determinable point during the extending motion of the pumping piston to establish and disestablish a hydraulic link between the pumping piston and the metering piston to initiate fuel injection, and with (c) the metering chamber for permitting fuel to flow into the metering chamber after the time that said pumping plunger has begun its retracting motion. The invention further comprising a plurality of fluid passages interconnecting the control valve means, the pressure source, the timing chamber and the metering chamber; wherein the retracting motion of the pumping piston is halted at a determinable position in its upward travel during the time that fuel is permitted to flow into the metering chamber, and wherein the pumping piston is maintained at this determinable position until the time that the timing chamber is opened to disestablish the hydraulic link.